Emissions legislation in Europe and North America has driven the implementation of exhaust aftertreatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide emissions (NOx) that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust aftertreatment technologies have been developed that treat NOx under these conditions.
One of these technologies includes a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR). Ammonia is difficult to handle in its pure form in the automotive environment, therefore it is customary with these systems to use a diesel exhaust fluid (DEF) to facilitate the reduction of NOx emissions. Typically, DEF is a liquid aqueous urea solution, usually at a 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue®. The urea is delivered to the hot exhaust stream typically through the use of an injector, and is transformed into ammonia prior to entry in the catalyst. More specifically, the urea is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2), the ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
In a diesel engine system, DEF is sprayed into the exhaust in front of a mixer and catalyst to convert NOx into Nitrogen and water vapor. The delivery device is called a Reductant Delivery Unit (RDU). Mounting the RDU to the exhaust pipe close to the engine rather than further downstream is referred to as a “close coupled system.” Close coupled systems are able to be used sooner after engine startup due to their proximity to the engine. DEF is not effective until exhaust temperatures reach approximately 150°-200° C. An RDU closer to the engine means higher temperatures are reached sooner, thereby reducing NOx sooner.
Close coupled systems typically use engine coolant to maintain acceptable RDU temperatures by passing the engine coolant through the RDUs. If a coolant system is not present, the ambient temperatures reached in a closed coupled system is likely to damage the RDU. Systems not considered as close coupled systems are mounted further away from the engine and only require airflow to maintain RDU temperatures.